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Study of gaseous emissions in the inner coma of the three H-Comets: P/Halley, C/1996 B2 Hyakutake and C/1995 O1 Hale-Bopp

These last 15 years, the study of three comets improved our understanding of the cometary medium: Comet Halley with its in-situ study in 1986, Comet Hyakutake that closely approached the Earth in 1996 and the very active Comet Hale-Bopp in 1997.

igh spatial resolution spectra of Comet Halley were obtained during the Vega 2 mission on March 1986. In the near-UV, several emission bands appear at cometocentric distances lower than 3000 km. One of them, centered on 323 nm, cannot be explained by any molecule usually observed in comets. It was also detected on one spectrum the day before the encounter 350000 km away from the nucleus, suggesting that the molecules responsible for this emission were released locally, probably by grains. Spectra in the liquid phase suggested that a polycyclic aromatic hydrocarbon (PAH), the naphthalene, could be a candidate. We made an experiment to obtain fluorescence spectra of naphthalene under conditions that provide valuable simulation of the cometary situation. The result, a large band centered on 350 nm, cannot explain the emission band observed at 323 nm that remains unidentified.

Spectra of Comet Hyakutake were obtained with the IUE satellite in February and March 1996. Its close approach to the Earth gave the opportunity to observe with a high spatial resolution and to investigate the inner coma where the short-lifetime molecules released by the nucleus are concentrated. This is the case of S2 with a lifetime of 250 s that emits in the spectral region recorded by IUE. We elaborated a time-dependent fluorescence model of S2 that showed that the molecules reach a state very close to the steady-state within 200 s only. The comparaison between our synthetic spectrum and the IUE spectrum of Comet IRAS-Araki-Alcock in which S2 was discovered showed that the molecules are close to the steady-state. In comets with a relatively low density in the inner coma, such as comets IRAS-A-A and Hyakutake, the collisions in the inner coma do not slow down the fluorescence process and the molecules have time to reach a state close to the steady-state before being photodissociated. The comparison of the steady-state spectrum with a IUE spectrum of Comet Hyakutake obtained two days after the closest approach to the Earth with a long exposure time of one hour suggested that the observed and calculated spectra may be correlated. We derived an upper limit to the abundance of 0.01 % relative to water. This detection was confirmed five days later by observations with the satellite HST.

The brightness of comets Hyakutake and Hale-Bopp allowed us to make interesting optical observations at the Observatoire de Haute Provence. Different image processing revealed several structures in the comae. In Comet Hyakutake, there are dust jets in the solar side, two condensations along the tail and a crescent-shape arc present in the C2 emission between the nucleus and the first condensation. The condensations remained several weeks alined in the tail, suggesting that they are dust clusters released from the nucleus and not solid fragments. In Comet Hale-Bopp, the structures are dust arcs regularly spaced in the sunward hemisphere and C2 arcs that extend in the nightside hemisphere. Our program was partly based on narrow-band filter imagery. Two filters were used to isolate the C2 (1,1) and (0,0) bandheads at 513 and 516 nm and two others to measure the dust continuum at 527 nm and at 682 nm. The I(513nm)/I(516nm) intensity ratio gives the degree of excitation of the C2 molecules. I(682nm)/I(527nm) gives the dust color and characterizes the size distribution of the dust. The study of these two parameters revealed the presence of diffuse sources where the C2 molecules have a high excitation degree. They are produced by the photolysis of molecules not released by the nucleus but locally released by dust particles. In Comet Hyakutake, the diffuse source is between the nucleus and the first condensation, and is probably linked to the liberation of the condensations. In Comet Hale-Bopp, it corresponds to the concentric arcs in the sunward hemisphere.

In-situ observations of Comet Halley and observations of the bright comets Hyakutake and Hale-Bopp increased our knowledge of the cometary medium but numerous problems will remain non-resolved before spatial probes explore the nucleus of comets.